The ambitious agreement is challenged as indications arise that the Syrian government will not submit a toxic-stockpile inventory this weekend.

WASHINGTON — The ambitious U.S.-Russian deal to eliminate Syria's chemical weapons, hailed as a diplomatic breakthrough just days ago, hit its first delay Wednesday with indications that the Syrian government will not submit an inventory of its toxic stockpiles and facilities to international inspectors by this weekend's deadline.

The State Department signaled that it would not insist that Syrian President Bashar Assad produce the list Saturday, the end of a seven-day period spelled out in the framework deal that Washington and Moscow announced last weekend in Geneva.

Marie Harf, a State Department spokeswoman, said Wednesday that "our goal is to see forward momentum" by Saturday, not the full list. "We've never said it was a hard and fast deadline."

It wasn't clear whether Syrian officials needed more time to complete a formal declaration of their chemical arms, or whether the disarmament deal itself was in trouble.

Secretary of State John F. Kerry had described the date as the first of several "specific timelines" that would indicate whether Syria is committed to the deal that he and Russian Foreign Minister Sergei Lavrov had worked out.

"We agreed that Syria must submit within a week — not in 30 days, but in one week — a comprehensive listing," Kerry said Saturday. He said the U.S. would allow "no games, no room for avoidance, or anything less than full compliance."

Senior Obama administration officials had praised Russia for persuading Assad's government to relinquish its lethal chemical arsenal, one of the world's largest, by mid-2014 in a deal to avoid U.S. missile strikes in retaliation for the Aug. 21 poison gas attack that the U.S. says killed more than 1,000 people.

But Moscow's ability or willingness to push its ally in Damascus to meet the first deadline in the deal now is being questioned.

Kerry and Lavrov sought last weekend to portray the two powers as united. The gap between them, however, has become more apparent and is threatening to snarl efforts to craft a United Nations Security Council resolution that lays out how Syria is to meet its obligations.

The resolution needs to be complete before the first steps can be taken to impound and either remove or destroy Syria's arsenal. Diplomats said Western countries split with Russia in a meeting Tuesday over Western demands for tough enforcement of the agreement.

Diplomats hope to complete the resolution by Friday, but if they fall short the work may be delayed further next week because of the meeting of the U.N. General Assembly.

The Organization for the Prohibition of Chemical Weapons, an international body based in The Hague, is expected to take several days to complete its analysis of the Syrian "initial declaration," and then will submit its report to the United Nations.

Gary Samore, who was President Obama's top arms control advisor until February, said the declaration is key because "it will provide an early test of whether this process is ever going to get off the ground."

Samore, now research chief for the Belfer Center at Harvard's Kennedy School of Government, said he believes Russia is pushing Syria to comply. He also said, "Assad is going to try to hide some portion — maybe 10%, maybe 30% — whatever he thinks he can get away with."

Western diplomats close to the deliberations at the U.N. are wary that the Syrians may try to "cheat and retreat," as Saddam Hussein's government did for years in Iraq, to stymie U.N. weapons inspections.

"We're not going to lose a lot of faith in the Syrians, because we're not starting out with a lot," said one diplomat, who declined to be identified because of the sensitivity of the issue.

Western diplomats said they wanted to avoid a mistake of the U.N.'s battle with Hussein by starting out with an enforcement mechanism strong enough to prevent the Syrians from avoiding their obligations.

The Russians and Americans are sharply split over the use of the U.N.'s Chapter 7, which authorizes punitive actions, in any resolution. Russia has argued that it should be included only as a possible avenue for future action, while the United States, Britain and France want it conveyed automatically if there is noncompliance.

Russia also amplified its claims that rebels seeking to overthrow Assad, not the Syrian government, fired rockets filled with deadly sarin gas Aug. 21, and described a U.N. report on the incident as "politicized" and "one-sided."

U.N. spokesman Martin Nesirky defended the report Wednesday, calling it "indisputable" and "thoroughly objective." Independent groups have analyzed data in the report and concluded that the rockets were fired from government-controlled areas into territory held or contested by rebels.

U.S. officials say they have seen no evidence that chemical weapons are stored or being used in areas held by the opposition.

Some experts argued that the one-week timeline was too generous. Robert M. Gates, former Defense secretary under George W. Bush and Obama, said this week that Assad should have been given a 48-hour ultimatum.

"It should be an easier task on the part of the Syrians compared to the Iraqis," said Charles Duelfer, who led the U.S. search for weapons of mass destruction after the 2003 invasion of Iraq.